Added new virtual reseveElements, setElement, getElment and addElement methods to DrawElements

to make is easier to write code that can work on DrawElementUByte, UShort or UInt.

Changed the osgTerrain::GeometryTechnique so that it automatically chooses 
the use of DrawElementUShort or DrawElementsUInt accordining to the size of the tile.

src/osgPlugins/ffmpeg/FFmpegDecoder.cpp

@@ -54,10 +54,15 @@ bool FFmpegDecoder::open(const std::string & filename)
 
             formatParams.channel = 0;
             formatParams.standard = 0;
+#if 1
             formatParams.width = 640;
             formatParams.height = 480;
+#else
+            formatParams.width = 640;
+            formatParams.height = 480;
+#endif            
             formatParams.time_base.num = 1;
-            formatParams.time_base.den = 50;
+            formatParams.time_base.den = 15;
 
             iformat = av_find_input_format("video4linux2");